Add centralized error messages and validation utils

Introduce errorMessages.js with i18n translation keys for all
user-facing errors (form, connection, API) and validators.js with
reusable validation functions that reference those keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/constants/errorMessages.js

@@ -0,0 +1,25 @@
+// Translation keys for all error messages.
+// Values must match the keys defined in src/locales/de/translation.json.
+// Callers pass these to t() — e.g. t(ERROR_MESSAGES.form.usernameRequired).
+
+export const ERROR_MESSAGES = {
+  form: {
+    usernameRequired: 'error.form.usernameRequired',
+    roomRequired: 'error.form.roomRequired',
+    usernameAndRoomRequired: 'error.form.usernameAndRoomRequired',
+    userIdRequired: 'error.form.userIdRequired',
+    messageTextRequired: 'error.form.messageTextRequired',
+  },
+  connection: {
+    failed: 'error.connection.failed',
+    noActiveConnection: 'error.connection.noActiveConnection',
+    interrupted: 'error.connection.interrupted',
+    sendFailed: 'error.connection.sendFailed',
+  },
+  api: {
+    joinFailed: 'error.api.joinFailed',
+    noUserReturned: 'error.api.noUserReturned',
+    snapshotFailed: 'error.api.snapshotFailed',
+    leaveFailed: 'error.api.leaveFailed',
+  },
+}

frontend/src/utils/validators.js

@@ -0,0 +1,25 @@
+import { ERROR_MESSAGES } from '../constants/errorMessages'
+
+export function validateRequired(value, message) {
+  if (!value || !String(value).trim()) return message
+  return null
+}
+
+export function validateJoinForm(username, room) {
+  if (!username.trim() || !room.trim()) {
+    return ERROR_MESSAGES.form.usernameAndRoomRequired
+  }
+  return null
+}
+
+export function validateUsername(username) {
+  return validateRequired(username, ERROR_MESSAGES.form.usernameRequired)
+}
+
+export function validateRoom(room) {
+  return validateRequired(room, ERROR_MESSAGES.form.roomRequired)
+}
+
+export function validateMessageText(text) {
+  return validateRequired(text, ERROR_MESSAGES.form.messageTextRequired)
+}